Parallel inverter with rapid response time to changes in pulse durations



Oct. 15, 1968 R. w. FRENCH I 3,406,329

PARALLEL INVERTER WITH RAPID RESPONSE TIME TO CHANGES IN PULSE DURATIONSOriginal Filed Feb. 28, 1964 2 Sheets-Shet 1 wAvE FORM 2h FIGJ A.C. LOADWAVE FORM Dc "$54 souRcs WAVE FORM WAVEZSORM ed A SCR3

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7 B B $.46 WAVE FORM 20 WAVE FORM 20 WAVE FORM 2c (WAVE FORM 2b T T ITISCRI TRIGGER I I T2 T scRz TRIGGER I I I FIG-2b I I I n T3 r D 4 r 0scRa a SCR4 TRIGGER I I I I T I 5| F|G.2c

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OUTPUT FIG-2h INVENTORZ RICHARD W. FRENC HIS ATTORNE Oct. 15, 1968 w, EH 3,406,329

PARALLEL INVERTER WITH RAPID RESPONSE TIME TO CHANGES IN PULSE DURATIONSOriginal Filed Feb. 28, 1964 2 Sheets-Sheet 2 I F|G.3o T 2 I scRzTRIGGER I T i I FIG-3b T3 k-T T -T scRaascFm TRIGGER I H I l F|G.3c

s (0N) SCRYS ANODEO Q (a FlG.3d

5 \o +2v (OFF) SCRI ANODEO ----M F SCR4 ANODE 0 (0N) FlG.3f

+2v (OFF) SCR2 ANODE o 's- 9 OUTPUT WAVE FORM 1 l FlG.3h

INVENTORZ RICHARD W. FRENCH,

HIS ATTORNEY.

United States Patent 3,406,329 PARALLEL INVERTER WITH RAPID RESPONSETIME TO CHANGES IN PULSE DURATIONS Richard W. French, Liverpool, N.Y.,assignor to General Electric Company, a corporation of New YorkContinuation of application Ser. No. 348,002, Feb. 28, 1964. Thisapplication Dec. 19, 1966, Ser. No. 602,833 2 Claims. (Cl. 321-45) Thisinvention relates to parallel inverter circuits and more particularlyhas as an object to provide a parallel inverter circuit adapted fordiscontinuous or pulsed operation. The present application is acontinuation of U.S. application S.N. 348,002, now abandoned.

Inverters are devices which achieve a conversion of DC energy to ACenergy. In some applications they are accompanied by additional meansfor reconverting the AC energy back to DC energy. In principle,inverters depend upon one or more switching devices usually of the typeknown as controlled rectifiers to interrupt the flow of energy from asource of direct current energy to a load. When a pair of switchingdevices are employed, one can successively reverse the direction ofcurrent flow through a load and thereby supply an alternating currentthereto. The parallel inverter is of this nature, and has a pair ofswitching devices, each controlled to operate alternately, in each oftwo separate paths energized in parallel by the DC source. Energyalternately available in the separate primary paths may be coupled asalternating current energy to the load by a transformer having a pair ofprimary windings feeding a single secondary load windmg.

Inverters are inherently capable of proportioning or regulating thepower or load current supplied to a load and may be used over a widerange of power levels. One mechanism for effecting such control is thatof adjusting the duration of the conduction time of the switchingdevices. Assuming the constant output frequency, one can then adjust thefraction of the time for each half cycle that each switching device isconducting to bring about a control of the power current supplied to aload.

The switching devices or controlled rectifiers in such inverters maytakethe form of thyratrons or ignitrons or solid state devices of the typenow referred to as Silicon Controlled Rectifiers (SCRs). These devicesare all threeelement devices having in common a high and a low state ofconductivity, i.e., an on and an o state. They are usually turned on,once anode potentials have been applied, by a pulse applied to thecontrol element. Due to the nature of these devices, turning offrequires removal of the anode potentials and is not convenientlyachieved by adjustment of the control potentials alone. Often to enhancerapidity of turn-01f, inverse anode, or inverse anode and inversecontrol potentials are applied.

The thyratrons or the ignitrons operate at high power levels and areusually characterized by rather slow switching speeds. The maximum powercapabilities of SCR devices are still being expanded but at the momentthey are less than those of the thyratrons and ignitrons. However,because of the nature of [the conduction mechanism, SCR devices do havethe capability of operating at lower supply voltages and at higheroutput frequencies than the former classes of devices. In applicationswhere both classes of devices can be employed, operating efficiencyoften favors the SCR devices which exhibit lower dissipation losses.

The conventional parallel inverters do not permit convenientdiscontinuous or pulsed operation. Due to the nature of the controlledrectifiers, as explained above, mere removal of the control pulses willhave the effect of leaving one or the other in a state of prolongedconduction. Efficiencies in AC generation dictate that the primarycircuit be of low DC impedance. The circuit usually comprises thecontrolled rectifier and the customary transformer primary, both ofwhich are connected in series across the DC source. Consequently afailure of one of these rectifiers to cycle, as by removal of thecontrol pulses, permits the current in the primary circuit whose how isrestricted primarily by reactive impedance, to surge to and remain at amaximum, limited only by the DC irnpedances in the primary circuit. Evenwere it not a question of catastrophic circuit failure, electricalefficiency dictates that power dissipation in the primary circuit beheld to a minimum in the intervals when no power is being fed to a load.

The expedient of directly switching off the direct current source fromthe primary circuit in order to protect the primary circuit in pulsedoperation has several disadvantages. It requires an additional highcapacity switching means which is a major disadvantage. It also requiresadditional circuit complexity if one wishes to achieve rapidity inresponse in efiecting control of the pulse envelope durations; or toretain, in starting and terminating the pulse envelope, the sameaccuracy in timing of the moments of turn-on and turn-off of theindividual cycles achievable in the controlled rectifiers themselves.

It is therefore an object of the present invention to provide a parallelinverter system capable of pulsed or intermittent operation which doesnot require additional high current level switching means.

It is still another object of the present invention to provide aparallel inverter capable of pulsed operation wherein the circuit has aminimum of energy dissipation in the intervals between the delivery ofenergy to a load.

It is still another object of the present invention to provide aparallel inverter circuit capable of pulsed operation with a rapidresponse time to accommodate rapid changes in pulse envelope durations.

It is a further object of the invention to provide an improved parallelinverter system capable of pulsed operation having high accuracy intiming of both the moments of turn-on and turn-off of the individualhalf cycles of conduction as well as in the moments of initiation andtermination of the overall p'ulse envelope.

Briefly stated, these and other objects of the invention are achieved ina parallel inverter circuit adapted to be connected to a source ofdirect current energy and employing a transformer, a pair of controlledmain power rectifiers, and including a pair of novel switching circuits,each associated with one of said main power rectifiers. The transformerhas a tapped first winding and one anode of one rectifier is connectedto one end of the winding and the other anode of the other rectifiercoupled to the other end of the winding. The negative terminal of thesource of direct current energy may then be coupled to the cathodes ofthe main power rectifiers and the positive terminal to the tap of thetransformer winding. A load may be inductively coupled to the mainwinding.

In accordance with one embodiment of the invention, the novel switchingcircuits so provided, each include an auxiliary controlled rectifier; acommutating capacitor, which is coupled between the anodes of the mainrectifier and the auxiliary rectifier; and a resistance, which iscoupled between the anode of the auxiliary controlled rectifier and thepositive source terminal. The cathode of the auxiliary controlledrectifier is coupled to the negative source terminal. Each of the fourcontrolled rectifiers in the overall circuit are then provided withsuitable control voltages to elfect proper sequential operation of themain power rectifiers. In achieving this function, the main rectifiersare turned on by suitable control potentials applied to their respectivegates, and turned off by the switching circuits as a result of controlvoltages applied to the auxiliary rectifiers.

In a preferred version of, the foregoing embodiment, a resonatinginductor connected in series with a diode is coupled in shunt with theelectrodes of the auxiliary controlled rectifier, with the diode beingpoledto pass current from ground into the inductor. The effect of thisprovision is to enhance the efficiency of commutation.

The subject matter of the invention is more particularly pointed out anddistinctly claimed in the concluding portionof the specification. Theinvention, however, may best be understood by reference to the followingdescription taken in connection with the following drawings in which:

FIGURE 1 illustrates a first embodiment of the invention.

FIGURES 2a through 2h illustrate the waveforms existing at variouscircuit points of the first embodiment. The waveforms are plotted asvoltage vs. time'with common time coordinates.

FIGURES 3a through 3h illustrate the waveforms existing at variouscircuit points of a second embodiment, representing a modification ofthe first embodiment.

In the parallel inverter circuit of the invention shown in FIGURE 1,there are provided first and second controlled main power rectifiersSCR1 and SCR2 and first and second switching circuits including,respectively, first and second resonant charging circuits and first andsecond controlled auxiliary or turn-off rectifiers SCR3 and SCR4. Eachof the SCRs includes an anode A, a cathode C, and a gate G. The anodes Aof SCR1 and SCR2 are connected respectively to opposite end terminals 1and 2 of primary winding P of transformer T, center tap terminal 3 ofthe transformer primary winding P being connected to a positive terminalof DC power supply 4. The transformer T is provided with a secondarywinding S coupled to a load 14. The cathode terminals C of SCR1, SCR2,SCR3, and SCR4 are connected directly to a negative terminal 5 of the DCpower supply. The first resonant charging circuit comprises, a capacitor6, a diode 7, and an inductor 8, respectively connected in seriesbetween the anode terminal A and cathode terminal C of SCR1. The secondresonant charging circuit comprises, a capacitor 9, a diode 10, and aninductor 11, respectively connected in series between the anode terminalA, and the cathode terminal C of SCR2. The diodes 7 and are poled foreasy current flow from the inductors 8 and 11, respectively, toward thecapacitors 6 and 9, respectively. The capacitor 6 interconnects theanode terminals A of SCR1 and the corresponding SCR3; similarly,capacitor 9 interconnects the anode terminals A of SCR2 and thecorresponding SCR4. Current limiting resistors 12 and 13 connect theanode terminals A of SCR3 and SCR4, respectively, to the positive powersupply terminal 4 to provide a holding current for maintainingconduction thereof.

The gates G of the rectifiers SCR1, SCR2, SCR3, and SCR4 are coupledrespectively to the terminals 15, 16, 17, and 18. These terminals are inturn supplied with trigger pulses illustrated in FIGURES 2a, 2b, and 2c.In particular, trigger pulses applied to terminal 15 are illustrated inFIGURE 2a; trigger pulses applied to terminal 16 are illustrated inFIGURE 2b; and trigger pulses applied to terminals 17 and 18 areillustrated in FIGURE 20. These figures will be explained below.

To provide an alternating current output, SCR1 and SCR2 are controlledin accordance with the invention to become alternately conductive forsuccessive intervals of time. SCR1, when conductive, permits current tofiow fromthe positive power supply terminal through primary winding Pfrom the center tap terminal 3 to the end terminal 1 in a firstinterval, and SCR2, when conductive, permits current to flow from thepositive power supply terminal through primary winding P from the centertap terminal 3 to the end terminal 2 in the successive interval. ThusSCR1 and SCR2, when alternately operated, each saqeaza 15 conductsuccessive pulses of output current alternately through the first andsecond halves of the primary winding P. These pulses, which are thuscoupled inductively by transformer T to the secondary winding S inrespectively opposite senses, produce an output waveform of alternatingpolarity for energization of load 14. Thisoutputwayeform isas'illustrated in FIGURE 2h. y The novel control means by whichalternate conduction of the main power rectifiers SCR1 andSCR2 isachieved will now be. explained with further resort to FIGURES 2athrough 2h.-

It may now be understood that at some initial time, the DC source 4provides a positive potential coupled to point 3 of the primary oftransformer T. At some subsequent instant illustrated at the moment T1in FIGURE 2A, a turn-on trigger pulse is applied to the control gate ofSCR1. This causes SCR1 to become conductive and permits a rathersubstantial amount of current to flow'from the DC source 4 through theupper half 'of primary winding P in the direction from terminal 3 toterminal 1 and to ground through SCR1. The effect of the trigger uponthe voltage at the anode of SCR1 is illustrated in FIGURE 2e, where itmay be noted that the potential which had previously been a highpositive potential has now been reduced to near zero potential,indicating the initiation of a period of high conduction.

At a moment T2 later in time than T1, a trigger pulse is applied to thegate G of SCR2 as illustrated in FIGURE 2b. Assuming the existence of apositive potential at the anode of SCR2 at the moment T2, the SCR2becomes conductive and its anode potential falls suddenly to near zero,indicating the initiation of a period of high conduction. This is asillustrated in FIGURE 2g.

As observed earlier, the mechanism for turning on the main powerrectifiers SCR1 and SCR2 is accomplished at time intervals, as explainedabove, by the recurrent application of the trigger pulses illustratedrespectively in FIGURES 2a and 2b. The mechanism for turning 01f SCR1before SCR2 becomes conductive and the mechanism for turning oif SCR2before SCR1 becomes conductive requires a consideration of the switchingcircuits associated with each of the-main rectifiers.

The operation'of these switching circuits will now be undertaken.

To achieve turn-off of a solid state device, such as an SCR, a chargedepletion region must be developed at'the internal rectifying junction.The depletion region can be developed inherently by the SCR if theforward biasing voltage applied between the anode and the cathodeterminals thereof is removed for a suflicient length of time such as bydisconnecting the SCR from the power supply circuit. Such an expedientis impractical in a parallel inverter circuit, however, and thus asecond method, namely that of applying a reverse bias across the anodeand cathode terminals of the SCR must be employed. In the circuit of theinvention, the auxiliary controlled turnoff rectifiers SCR3 and SCR4, inconjunction with the first and second resonant charging circuits,provide the reverse biasing for achieving turn-off of SCR1 and SCR2,respectively. In addition, SCR3 and SCR4 are controlled independently ofthe conduction of SCR1 and SCR2 whereby both pulse-width modulation anda pulsed mode of operation are achieved in the parallel inverter circuitof the invention. I

To provide operation, suitable trigger pulses of a frequency equal tothe frequency of the desired output pulses are applied continuously tothe gate terminals G of SCR3 and.SCR4 whereby, in the absence ofconduction of SCR1 and SCR2, SCR3 and SCR4 are normally maintained in aconductive state. In the conductive state, the forward potential drop ofSCR3 and SCR4 essentially becomes that of a forward biased diode,whereby the capacitors '6 and 9 are subjected to a charging voltage Vsubstantially equal to the voltage drop across the non-conductingcontrolled power rectifiers SCR1 and SCR2, respectively, the

voltage V thus being approximately equal to the voltage of the powersupply, Vs. Following the current flow for charging of capacitors 6 and9 to the voltage V, SCR3 and SCR4 are maintained in the conducting stateby the forward holding current supplied from the positive power supplyterminal 4 and through the resistors 12 and 13, respcctively, to theanode terminals A of SCR3 and SCR4. The value of the forward holdingcurrent is given by the equation:

R (1) where i equals the forward holding current, Vs equals the value ofthe potential source supplied to the positive power supply terminal 4,and R equals the resistance value of either resistor 12 or resistor 13.Ideally, and for minimum power dissipation, the resistors 12 and 13 areof a magnitude which just permits the minimal value of forward holdingcurrent to flow. In practice, however, the resistors 12 and 13 will bemade slightly smaller so as to supply a forward holding current slightlyin excess of the requisite minimum forward holding current to assure.continued conduction of SCR3 and SCR4 in spite of transients which maybe introduced in the circuit and which might effect an undesirabletermination of conduction of SCR3 or SCR4. While SCR3 and SCR4 aremaintained conducting by the forward holding current z';,, the triggerpulses still are continuously applied to their gate terminals G,although they have no further effect.

To generate output power pulses, suitable trigger pulses are applied tothe gate terminals G of SCR1 and SCR2 in complementary fashion, SCR1 andSCR2 respectively generating, as hereinbefore mentioned, successivealternate half cycles of the output power pulses. The turn-off operationof the circuit will be described with reference to SCR1 and itscorresponding switching circuit comprising SCR3 and the first resonantcharging circuit, it being recognized that SCR2 and its correspondingswitching circuit comprising SCR4 and the second resonant chargingcircuit operate in an identical fashion.

The voltage V developed on capacitor 6 during conduction of SCR3 is of apolarity indicated in FIGURE 1. Upon triggering of SCR1 into conduction,its forward voltage drop is essentially that of a forward biased diodeand thus the anode terminal A of SCR1 is essentially at groundpotential. Since the charge on capacitor 6 cannot changeinstantaneously, and since the plate of capacitor 6 which is connectedto the anode terminal A of SCR1 is referenced essentially to groundpotential when SCR1 is turned on, the other plate of capacitor 6, whichis connected to the anode terminal A of SCR3, drives the anode terminalA of SCR3 negative in an amount equal to the potential V. Thus, turn-onof SCR1 effects commutation by capacitor 6 to turn off SCR3.

Diode 7 is forward biased by the voltage -V at its cathode and thus isin condition for conduction. Capacitor 6 becomes a potential source,forcing current through the now conducting SCR1, inductor 8, and diode7. Inductor 8 instantaneously develops a voltage to oppose this flow ofcurrent and thus has induced therein a potential of the polarityindicated in FIGURE 1. The value of the voltage developed across, orinduced in, inductor 8 is determined by the equation:

dt (2) where E equals the induced voltage, L equals the value ofinductor 8, and di/dt equals the rate of change of the current i withtime. Capacitor '6 continues to forcethe current 1' through the resonantcharging circuit until it is completely discharged, at which time thecurrent i is a maximum, and the voltage across both the capacitor 6 andinductor 8 is zero and that at the cathode of diode 7 is near zero.Since capacitor 6 now is discharged completely, the current begins todecrease, whereby di/dt, the

rate of change of current, reverses its sign. The potential induced ininductor 8 instantaneously reverses its polarity, as indicated byEquation 2 above, and assumes a polarity opposite to that indicated inFIGURE 1.

During the second half of the charging cycle the inductor 8 is thepotential source, forcing the current i through diode 7 and chargingcapacitor 6 to a polarity opposite to that indicated in FIGURE 1. Thecurrent i continues to be forced through the circuit by the inductor 8until capacitor 6 is completely charged, at which time the current i hasbeen reduced to zero, and the potential across both capacitor 6 andinductor 8 reach their maximum values and are of a polarity to cause thepotential at the cathode of diode 7 to become +V, reverse biasing diode7 to its non-conducting state. In an ordinary resonant circuit,capacitor 6 would become the voltage source and begin to force currentthrough the resonant charging circuit in the opposite direction.However, diode 7, now being reverse biased, prevents the reversal ofcurrent flow and the voltage condition stabilizes. Capacitor 6maintains, in substantially undiminished amount, the potential V for theremainder of the conducting interval of SCR1. To terminate conduction ofSCR1, an appropriate trigger pulse is applied to the gate terminal G orSCR3 to render it conductive. Due to the charge reversal on capacitor 6effected by the resonant charging circuit, capacitor 6 is charged nearlyto the voltage V of the requisite polarity to perform the commutationfunction for turning off SCR1.

As a part of the commutation function the capacitor 6 also must supplysufficient electrical energy to provide for the conduction of a reversecurrent through SCR1 and SCR3 to perform the turn-off function. Thereverse current aids in developing the depletion region and isproportional to the forward anode current at the time of turn-off. Sincethe forward current, i in SCR3 at time of turn-off is adjusted to bejust slightly greater than the minimum required holding current, thereverse current is quite small and effects only a slight reduction inthe total charge of potential V originally developed on capacitor 6. Theenergy required for turning off SCR1 is a great deal larger, however,due to the fact that turn-off of SCR1 may be required to be performed ata time at which SCR1 is conducting maximum load current. The energystored by the capacitor is calculated in accordance with the equation:

where U equals the amount of electrical energy, C equals the value ofthe capacitor 6, and V equals the voltage of the potential charge on thecapacitor 6. Since the energy required to be supplied by capacitor 6 isdetermined by the maximum turn-01f function to be performed, the valueof the capacitor 6 may be determined from Equation 3.

The waveforms for an ideal resonant charging circuit are essentiallysinusoidal; however, due to losses, the current i is partiallydissipated and the capacitor 6 is charged to a value less than theoriginal potential V. The actual potential attained at the cathode ofdiode 7 may be determined, assuming an original charge of V on capacitor6, and that the flow of current i recharges capacitor 6 to a value kV ofthe opposite polarity at time t=T The factor k is a function of the Q ofthe resonant circuit and increases as Q increases. By appropriate choiceof circuit components in the resonant circuit, a value of k of 0.8 to0.9 may be obtained, k ideally approaching a limit of 1.0. Some of thecharge developed on capacitor 6 also will be removed in supplying thereverse current through SCR3 when it is turned off by the conduction ofSCR1, as previously noted. Thus, the value of capacitor 6 should bedetermined by including the factor k in Equation 3.

The time T is the time in which the resonant circuit operates to achievea full polarity reversal, and may be calculated from the equation:

where L and C are the values of the inductor 8 and the capacitor6,respectively. T is thus one-half ofthe natural period of resonance.Inasmuch as the value of C is determined by the energy required forachieving turn-off of SCRl, primarily, and in accordance with Equation3, it will beappreciated that the value of the inductance L may bevaried for achieving a resonant'charging time, T of a desired value.

During the first conduction period of SCRl described above,autotransformer actiorrwill cause a voltage of 2V5 to be impressed uponthe anode of SCR2 and ms potential will cause capacitor 9 to be chargedto a higher potential (ideally 2Vs) than the potential Vs which itacquired during the quiescent period preceding the application of thefirst triggerto the gate of SCRl. When SCRl is turned off followingthisfirst conduction period, the associated resonant charging circuitwill be inactive as the inductor 8 and the diode 7 are shunted by theconducting -SCR3. The capacitor 6 will now be supplied current from thepower via the transformer'primaryhalf from terminals 3 to terminal 1 andthrough SCR3 to ground. As the capacitor is charged its voltage polaritywill be reversed and become that shown in FIGURE 1. If this chargingaction were allowed to continue the capacitor 6 would eventually becharged to the supply voltage Vs and the circuit would be in itsoriginal condition (before the application of the first trigger to thegate of SCRl). However, SCR2 will be triggered to the on state at theend of the time interval T indicated in FIGURE 2C following turn-oil ofSCRl and by autotransformer action the potential on transformer terminal1 will increase to 2Vs. Capacitor 6 will now charge to this increasedpotential of 2Vs. Following the initial half-cycle of operation thepotential on the capacitors 6 and 9 will attain a value of +2Vs and 2Vson alternate half-cycles as indicated by the waveforms of FIGURE 2,neglecting losses in the resonant charging circuit components.

It will be appreciated that the action of the resonant charging circuitcomprising capacitor 9, diode 10, and inductor 11, for effectingreversal of the charge pola ity on capacitor 9, will be identical tothat of the first resonant charging circuit but coordinated with theoperation of the corresponding SCR2 and SCR4 during generation of thesucceeding alternate half-cycle of the output power pulse.

The timing chart of FIGURES 2a to 2h, and in particular FIGURES 2dthrough 2g, illustrate the manner in which the corresponding rectifiersSCRl and SCR3 and the corresponding rectifiers SCR2 and SCR4 areoperated in the process of producing a series of output pulses. Onecomplete output pulse is generated within each of the successive timeperiods T as indicated by the FIGURE 2h. T is the period of one cycle ofthe output pulses, and is related to the frequency 1 thereof by theequation:

f Assuming that SCR3 and SCR4 have been appropriately triggered forcharging their respective capacitors 6 and 9, and further assuming thateach complete cycle of an output pulse commences with the initiation ofconduction of SCRl, the first half-cycle of an output pulse is generatedby the application of trigger pulse T1 as shown at FIG- URE 2a to thegate terminal G of SCRI. SCRl thereby conducts the power current fromthe positive terminal of power supply 4 and through the primary windingP from the center tap terminal 3 to the end terminal 1. The length ofthe conductive interval of SCRl is determined by, and essentially equalto the time interval following T1 at which trigger pulse T3 as shown at,FIGURE is applied to the gate terminal G of SCR3 to initiate itsconduction as shown at FIGURE 2d thereby terminating conduction of SCRl.The anode voltage of SCRI is shown at FIG- URE 2e.

For a given frequency of output power pulses, the time interval iscontrolled by two limiting factors. The first factor is that the timeinterval" of conduction of SCRl must be in excess of the time, T which,as represented 'by Equation 3, is a function of the resonant circuitparameters, to permit the charge polarity reversal on the cornmutatingcapacitor 6 to be effected. T may generally be madesmall relative to theperiod T of the output pulses and hence is usually a relativelyinconsequential limiting factor. The second factor relates to theautotransformer action of primary winding P due to the parallel connection and alternate conduction of SCRI and SCR2. When SCR2 is triggeredinto conduction by trigger pulse T2, as shown at FIGURE 212, current isconducted from the potential source through the positive terminal ofpower supply 4and the primary windingP for generating the secondhalf-cycle of the output pulse. The anode voltage of SCR2 is shown at2g. The autotransformer actionof primary winding P causes a rapid riseof voltage at the anode terminal Aof SCRl. A finite amount of time isrequired, however,'to develop the depletion region'at the rectifyingjunction of SCRl to achieve its complete turn-- off. If the chargedepletion region is not established, the positive rise of voltage at theanode due to the' fau to transformer action will cause conduction ofSCRIeven in the absence of a gate trigger. Trigger pulse T3, there fore,must occur sutficiently in advance 'of trigger pulse T2 suchthat therequisite charge depletion region for complete turn-oft" of SCRl isachieved prior to this rapid rise in anode voltage upon conduction ofSCR2/This interval is T as illustrated in FIGURE 20. The minimum timeinterval T following T at which T occurs likewise is controlled by theperiod T of the second resonant circuit, which is equal to the period TRof the first resonant circuit. Thus, the minimum time intervals T andT5", shown at FIGURE 2c'are equal.

Further, to achieve balanced output power pulsesias shown at 2h, thetime interval between T and T -mus't be identical to the time intervalbetween T and T that SCRl and SCR2 conduct for equal lengths of time.Thus, T and T are complementary to one another, occurring at identicaltime intervals following trigger pulses T and T respectively, and thusat identical t ime intervals following initiation of their respectivehalf-cycles of the output power pulses. I

The capability of providing pulsed operation in the parallel invertercircuit of the invention is a' direct result of the independentoperation of the switchingcircuits relative to either of the controlledpowerrectifiers'SCRl andSCR2. 'As discussed above, the trigger pulsesapplied to the various SCRs are necessarily synchronized to m videbalanced output' pulses. However, the trigger pulses T and T are appliedcontinuously to-SCRS and SCR4, respectively, thereby maintaining theirrespective switch.- ing circuits in acondition to perform their turn-olffunc; tions at all times independently both of each other'and of theconductive state of either of the main controlled power rectifiers.Further during the turn-on, of each main power rectifier, thecorresponding switching circuit may be energized at a desired earliertime during thecouductionim terval to effect an earlier turn-offthereof,to control the output pulse width for suchpurposes as regulatingthe output power. In the usual event this would correspond to anadvancing in time of all the triggers illustrated in FIGURE 20 appliedto the control rectifiers SCR3 and SCR4. The minimum duration pulsewidth is then limited bythe commutation rate of the resonant chargingcircuit as explained earlier and by the time actually required to turnoff the main rectifiers. This adjustment of conduction intervals is thepreferred method of achieving a regulation of the output power of theconverter. H

The circuit is instantly responsive to applied ,control pulses once theDC source is energized, placingthe circuit in stand-by condition.Initiation and termination of the output pulse envelope is effected bymerely starting and stopping the pulse trains applied to the main powerrectifiers. In stand-by condition, as explained earlier, each main powerrectifier is turned on substantially simultaneously with the applicationof its respective control pulse. Also, during stand-by condition; thecontrol'circuit, which is required for appropriate cyclical turn-ofi ofthe main rectifiers, is energized. Thus the coverter in stand-bycondition is' subject to'complete instantaneous control, and can soremain for any desired time interval.

The power dissipation of the converter during stand-by condition isquite modest. The main rectifiers (SCRl and SCR2) dissipate only a smallamount of'energy in the form of leakage current, since they are in anoff condition. The controlling rectifiers -(SCR3 and SCR4) and theirassociated circuits dissipate only modest amounts of energy since thecircuit can be adjusted to provide the minimum currentrequired to keepthese rectifiers in the high conduction state. The energy dissipationrequired for this function is also qiute small. l

In a typical application'of pulsed operation, the frequency of operationof the circuit may been the order of a kilocycle with pulse envelopes ofany duration. Since the commencement and termination of the pulseenvelope is controlled by the pulses used to initiate each individualpulse, it may be seen that one may achieve a high accuracy indetermining the pulse envelope initiation, termination, and durationcomparable to a fraction ofan individual pulse.

Due to the connection of the cathodes C of the rectifiers SCRl, SCR2,SCR3,'and SCR4 directly to the negative power supply terminal 5, thetrigger pulses may be applied directly to the gate terminals G of theserectifiers'by direct connection from any suitable triggering sourcewithout the need of employingcoupling transformers as required in manyprior art circuits. Further, since the trigger pulses T and T applied toSCR3 are required to be complementary, and since SCR3 and SCR4 are to becontinuously triggered, a single source of trigger pulses operating at afrequency twice that of the desired frequency of the output power pulsesmay be applied in common to the gate terminals G of both SCR3 and'SCR4.

In the circuit of FIGURE 1, a ballast inductor may also be providedhaving a first winding connected in series between the center tap 3 ofthe primary winding P and the positive terminal of power supply 4, and asecond winding also connected to the positive terminal of the DC sourcebut having its other terminal connected to the cathode of a diode. Theanode of the diode would be connected to ground. (The resistors 12 and13 would be retained coupled to the positive terminal of power supply4.)

The ballast inductor 19 and associated diode may be used to reducecurrent surges in the primary circuit. They are particularly useful whenthe load 14 is a tuned load or capacitive but may not be necessary whenthe load is inductive or resistive. The inductance of the first windingof the inductor is made sufiiciently large to stabilize the currentderived from the source throughout an individual pulse period. Thesecond winding on the ballast inductor and the diode reduces the voltageexcursions which would otherwise occur in the primary circuit as aresult of the inductance of the first winding. The second winding of theballast and the diode divert the current which would undesirablycirculate in the primary circuit of the transformer as the inductivefield of the ballast collapses, and returns the current to the powersupply.

This measure provides a substantial improvement in circuit efficiency aswell as a reduction in voltage transients on the SCR devices. Theaddition of the second winding limits the voltage at the center tap 3 ofthe primary winding P in accordance with the equation:

1 V Vs(1lwhere V is the voltage at the center tap 3, Vs is the voltageof the power supply 4 and N and N are the number of turns on the firstand second windings, respectively. A practical ratio of N to N may be 4to 1.

A parallel inverter circuit similar to that illustrated in 10 FIGURE 1and having a power output on the order of 1000 watts may have thefollowing typicalcircuit values:

Capacitors 6 and 9 1,2 ,uf., 600 v. Ballast inductor (when used)-; 1rnh. a. DC

p, I H -N /1y =4 Inductors 8 and 11 175. ,uhrg Diodes 7 and 10....;1N250. 7 t SCRI, SCR2 2N686. l

. SCR3, SCR4 e 2N1775A.

Resistors 12 and 13-. 16,0009. a Load Resistors 1859. DC source 80 v.

While the -output waveforrns have been shown 'to be slightly trapezoidalrectangular pulsesof alternating polar; ity, one can in fact produce avariety of output waveforms which vary from sinusoidal to nearly perfectrectangular pulses. Where it is desirable to produce a sinusoidal waveform, this can usually be achievedlby tuning' th e load to make itessentially resonant at the pulse fr'equencyi x In addition to producingalternating waveforms, one may rectify the output pulses to produce a DCoutput voltage. I I: l

The parallel inverter circuits of the invention are ex tremely useful inthat they provide both a pulsed mode of operation and means forregulation of the output power. In operation, the circuits are veryefli'cient, fsince only a minimal amount of power is dissipated in theswitching control circuits. Further, the independent control of theswitching co trol circuits and the main power rectifiersby externalgating signal means provides greater reliability and more versatility inregulation than can be achieved with prior art circuits. Theseadvantages are achieved in configurations which are simple inconstruction and economical of components. I

The parallel inverter embodiments so far described, may in certainapplications he further modified- Instead of utilizing a separatesecondary for load energization, one may couple the load directly totaps arranged on the primary winding in the manner of anautotransformer. For symmetry in output waveform, the taps should bearranged at equal distances fromthe centerof the primary winding. Inorder to eliminate a fixed DC potential in the load, one wouldordinarily earth the positive terminal of the DC source and operate itsnegative terminal below ground potential.

One may eliminate the resonating inductor and associated diode in bothbranches of the configuration of FIGURE 1. This has the effect ofreducing the commutating eificiency of the circuit. When the circuit isoperated at a slow rate, the commutating loss may be a small fraction oftotal energy delivered and thus quite tolerable. However, as thecommutating rate increases, the energy loss increases proportionately,since the loss is essentially constant for each cycle of operation.

The commutating action takes place in this modified circuit in thefollowing fashion. (Reference, for convenience will be made to FIGURE 1,where now the inductors 8 and 1 1 and associated diodes 7 and 10- areremoved.) The waveforms which appear at various points in the circuitare shown in FIGURES 3a-3h. Let us further assume an initial operatingcondition in which the SCR3 is on, SCRI is off, and the capacitor 6 hasbeen charged to 2Vs. Due to autotransformer action of the full primarywinding, the voltage across the capacitor will be double Vs and willhave the polarity shown in FIGURE 1.

Let one now trigger SCRl on. The charge corresponding to 2Vs is usefullyused to extinguish the auxiliary rectifier SCR3, which accounts for avery small portion of the total capacitor energy, and the balance isessentially dissipated in resistor 12. Current continues to flow 'in thesame direction through capacitor 6 and resistor 12 until the capacitorhas been charged to the voltage Vs in the opposite polarity to thatshown in FIGURE 1. The waveform at the anode of SCR3 is shown in FIGURE3a.

Essenti l y,-cal goi. t e. i iti ene y oi th -9ap ci /2 C(2Vs )=2CVs isdissipated in discharging the' capacitor through resistor 12, andfduringthe charging cycle as rnuch energy is dissipated in the serial resistor12 as is placed in the capacitor 6 /2C Vs v To complete this half-cycle"of operation, SCRS is now fired, while the capacitor 6 is charged tothe voltage Vs in a polarity opposite to that shown in FIGURE 11 Asubstantial fraction of the energy stored in the capacitor may berequired to extinguish the rnain SCRl, the fraction being dependent onits level of conduction. The remainder of the energy is'then used toenhance current flow through the primary winding of transformer T, thuscont'r'ib utin g to the total output power. Current continuesin the samesense through the capacitor 6 and SCRS to ground until capacitor 6 ischarge to 12Vs in a polarity shown in FIGURE I, This charging path hasrelatively little loss." r I "j The charging cycleis now complete withapproximately [3CVs units of energy being jexpendecl' to charge thecommutating capacitor and only approximately [VzL'CVs] units of enengybeing usefully used to' turn 01f the'rectifiers. Thus it follows thatmore than /6 of the energy accumulated in the capacitor during theco-mrnutating cycle is unused.

Although the invention has been described with respect to certainspecific embodiments, it willbe appreciated that various modificationsand changes may be made by those skilled'in theart without departingfrom the spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent inthe United States is:

1. In a parallel inverter for conversion of DC to AC electrical energy,the combination comprising:

a transformer having'a tapped winding;

a first and a second controlled rectifier each having anode, cathode andcontrol electrodes, the respective anodes of said rectifiers beingcoupled to opposite ends of said winding;

'input terminals for connection to a DC source being coupledrespectively to said tap and to the cathodes of said first and secondcontrolled rectifiers;

means for coupling to said respective control electrodes l2 .Q adfirst-aud se o d. c n o d- .rq ifiqr ti potentials at desired momentsin time;

a first switching circuit for extinguishing said first controlledrectifier comprising a first auxiliary controlled rectifier having ananode, cathode and control electrodes a first commutating. capacitor.coupled :between the anodes of said firstlrectifier and said auxiliaryrectifier, and meansinterconnec'ting the cathodes of {said two lastrecited rectifiers;

a second switcliing'circuit for'extinguishing -'-said second controlledrectifier comprising a 'secoi'n'd' auxiliary controlled rectifierfhaving an anode, cathode and control electrodes, a second "com-mutatingcapacitor coupled between the anodes ofi said second rectifier and saidsecond auxiliary rectifier, and means interconnecting the cathodesbfsaid two last recited rectifiersy a,

means for coupling to said respective, control electrodes of said firstand second auxiliary controlledirectifiers firing potentials at desiredmoments time; and energizing means including aresistancelfcoupledbetweensaid 'tapconnected input terminal and th eanode of each auxiliary rectifier to provide" current sufficient tosustain conduction therein. I

2. Thecombination set forth in claim 1 wherein each of saidswitchingcircuits additionally comprises energizing' means including a seriallyconnected resonating inductor and diode connected in shunt with theanode and cathode of said respective auxiliary rectifier, with the diodepoled to permit current flow toward said auxiliary rectifier anode. I

ReferencesCited UNITED STATES PATENTS" 3,075,136 1/1963 Jones 321 153,181,053 4/1965 Amato 32'1-45 3,213,352 10/1965 Faith 3'21- 4'53,263,153 1 7/1966 Lawn '321+45 3,349,314 10/1967 Giannamore 321j 43 LEET. HIX, Primary Examiner;

G. GOLDBERG, Assistant Examiner.

1. IN A PARALLEL INVERTER FOR CONVERSION OF DC TO AC ELECTRICAL ENERGY,THE COMBINATION COMPRISING: A TRANSFORMER HAVING A TAPPED WINDING; AFIRST AND A SECOND CONTROLLED RECTIFIER EACH HAVING ANODE, CATHODE ANDCONTROL ELECTRODES, THE RESPECTIVE ANODES OF SAID RECTIFIERS BEINGCOUPLED TO OPPOSITE ENDS OF SAID WINDING; INPUT TERMINALS FOR CONNECTIONTO A DC SOURCE BEING COUPLED RESPECTIVELY TO SAID TAP AND TO THECATHODES OF SAID FIRST AND SECOND CONTROLLED RECTIFIERS; MEANS FORCOUPLING TO SAID RESPECTIVE CONTROL ELECTRODES OF SAID FIRST AND SECONDCONTROLLED RECTIFIERS FIRING POTENTIALS AT DESIRED MOMENTS IN TIME; AFIRST SWITCHING CIRCUIT FOR EXTINGUISHING SAID FIRST CONTROLLEDRECTIFIER COMPRISING A FIRST AUXILIARY CONTROLLED RECTIFIER HAVING ANANODE, CATHODE AND CONTROL ELECTRODES,F A FIRST COMMUTATING CAPACITORCOUPLED BETWEEN THE ANODES OF SAID FIRST RECTIFIER AND SAID AUXILIARYRECTIFIER, AND MEANS INTERCONNECTING THE CATHODES OF SAID TWO LASTRECITED RECTIFIERS; A SECOND SWITCHING CIRCUIT FOR EXTINGUISHING SAIDSECOND CONTROLLED RECTIFIER COMPRISING A SECOND AUXILIARY CONTROLLEDRECTIFIER HAVING AN ANODE, CATHODE AND CONTROL ELECTRODES, A SECONDCOMMUTATING CAPACITOR COUPLED BETWEEN THE ANODES OF SAID SECONDRECTIFIER AND SAID SECOND AUXILIARY RECTIFIER, AND MEANS INTERCONNECTINGTHE CATHODES OF SAID TWO LAST RECITED RECTIFIERS; MEANS FOR COUPLING TOSAID RESPECTIVE CONTROL ELECTRODES OF SAID FIRST AND SECOND AUXILIARYCONTROLLED RECTIFIERS FIRING POTENTIALS AT DESIRED MOMENTS IN TIME; ANDENERGIZING MEANS INCLUDING A RESISTANCE COUPLED BETWEEN SAID TAPCONNECTED INPUT TERMINAL AND THE ANODE OF EACH AUXILIARY RECTIFIER TOPROVIDE CURRENT SUFFICIENT TO SUSTAIN CONDUCTION THEREIN.